rk3399: boot from compressed Image

[Tian Yuanhao]

Hello,

booti now supports booting from compressed Image.
See 414c34ed555b8ce5c260cf641261ecf45beca251.

Would you please add kernel_comp_addr_r and kernel_comp_size to
rk3399_common.h, so that people can use compressed Image (e.g. Image.gz)
in extlinux.conf.

Thanks,
Yuanhao